The HELLP syndrome is a serious complication of pregnancy characterized by hemolysis (H), elevated liver (EL) enzymes, and low platelet (LP) count that occurs in 0.2-0.6% of all pregnancies and in 10-20% of cases with severe preeclampsia and frequently leads to adverse maternal and perinatal outcome. The exact pathobiology of HELLP syndrome has not been clearly defined. As it is considered a form or a complication of severe preeclampsia, it likely has its origin in aberrant placental development and function resulting in ischemia-producing oxidative stress. However, there is still a debate on whether HELLP must be considered a severe form of preeclampsia or a separate disease entity. It can be described as a placenta-induced disease, as is preeclampsia itself, but with a more acute and predominant inflammatory process typically targeting the liver and with a greater activation of the coagulation system. This occurs during a disordered immunologic process and may be due to a genetic predisposition. In this review, we discuss the main biochemical characteristics of HELLP syndrome, particularly focusing on molecular aspects of placental involvement and maternal systemic responses.